Titanium dioxide (TiO2) could be doped with as much as 8.7 atom% Cr by means of a sol-gel method. XRD analysis revealed that the powder of Cr (8.7 atom%)-doped TiO2 calcined at 500degreesC consisted of small crystallites ascribale to anatase structure. The thin film of doped TiO2 (70 mum) at this composition was found to behave as a p-type semiconductor on exposure to CO and NO2 in air: it responded to dilute NO2 with a sharp decrease in electrical resistance.